Queueing outside Celtic’s pitch (7)
I believe the answer is:
incline
'pitch' is the definition.
('incline' can be a synonym of 'pitch')
'queueing outside celtic's' is the wordplay.
'queueing' becomes 'inline'.
'outside' indicates putting letters inside.
'celtic' becomes 'c'.
'inline' going around 'c' is 'INCLINE'.
(Other definitions for incline that I've seen before include "Bend forwards" , "List" , "Gradient" , "Lean (towards)" , "prejudice" .)